The rapid proliferation of portable electronic devices such as mobile phones, laptops, Personal Desktop Assistants (PDAs), portable media players and various electronic sensors and devices, has tremendously increased the demand for portable or unwired electric power. Generally, such devices are powered by rechargeable batteries. Examples, of rechargeable batteries include Lithium-Ion (Li-ion), Nickel-Metal Hydride (NiMH), Nickel Zinc (NiZN), and so forth. Typically, such batteries are charged by using charging devices such as commonly known Alternating Current (AC) adaptors. However, a charging device requires an external power source to charge a battery or power an electronic device. Moreover, various rechargeable battery chemistries are unstable at elevated temperatures. Therefore, such batteries may be required to be charged at a slow rate, while maintaining the charge accumulated during the charging.
Various energy harvesting (or “power scavenging”) technologies are known for generation and storage of electricity from mechanical vibrations of objects. Moreover, energy harvesting can provide an alternative solution, which is renewable and could conceivably not require replacement during the lifetime of the device. However, the small amount of energy available from the ambient environment and the low efficiency of most energy harvesting schemes have limited the application of these technologies to large wireless sensor nodes having power consumption of a few micro-Watts (M). The energy harvesting technologies may use a piezoelectric element such as a piezoelectric bimorph cantilever to generate energy from the vibrations of the mechanical objects. The piezoelectric bimorph cantilever can generate a useful voltage when it is deflected. Also, various other configurations of piezoelectric elements such as trapezoidal, cylindrical or conical can be used to increase energy output.
FIG. 1 illustrates a typical prior art arrangement of a piezoelectric bimorph cantilever 102. As shown, when a mass is attached to a free end 104 of cantilever 102 and input vibrations are provided at a base 106, then free end 104 with a mass 110 is deflected. Further, for piezoelectric materials layers 108a and 108b, the voltage (Vout) generated at electrodes is approximately proportional to the strain in piezoelectric materials layers 108a and 108b. Therefore, various energy harvesting devices are frequently designed relatively large for thin film devices (in order of centimeters), to accommodate a large proof mass (such as mass 110) and cantilever 102 with low stiffness. However, larger piezoelectric bimorphs are generally stiffer than smaller piezoelectric bimorphs. Furthermore, in various practical applications, the typical ambient vibration frequencies are very low, and/or multiple frequencies are present in the ambient environment, therefore the performance predictions for energy harvesting devices designed are unsatisfactory.
In light of the above discussion, techniques are desirable for efficient harvesting of energy for charging batteries and/or providing power to the electronic devices.